The present invention relates to a device for measuring the gradient of a magnetic field utilizing the action of a magnetooptical medium on a polarized light beam, known as the Faraday effect.
Such a device, which will be referred to hereinafter as magnetic field gradientmeter measures the variation in space of the module of the magnetic field, or one of its components in a given direction. If h.sub.1 and h.sub.2 are values of the components of the field measured at a distance d in said direction, the component of the gradient is given by the linear approximation: EQU (h.sub.2 -h.sub.1)/d
which applies for a spatial slow variation of the field with respect to distance d.
According to the prior art, the gradientmeters are produced by combining two magnetometers from which the two output signals are taken. The practical limit of the measurement of a magnetic gradient is then imposed by the precision of the alignment of the axes of these two pickups, and by the accuracy of the measurement of each pickup, the distance d being given. For example, if it is wished to measure a variation of 1 nT of the east-west component of a natural magnetic anomaly which is small compared with the earth's field (5.10.sup.4 nT in Europe), the parallelism of the measurement must be effected at better than 4 angular seconds.
It is known to produce very highly performing low weight gradientmeters, at least with respect to the sensitive part, by using SQUID (Superconducting Quantum Interference Device). They suffer from the disadvantage of having to be kept at a very low temperature of approximately 4.degree. K. and the associated cooling device is prohibitive from the consumption and overall dimensions standpoints.
Gradientmeters with a low consumption and accpetable overall dimensions have been produced by using two magnetometers of the saturatable core type, called "fluxgate", which are connected differentially. Although the local measurement of the field is sufficiently accurate for measuring low gradients, e.g. 1 nT/m, these gradientmeters suffer from the disadvantage of having a not sufficiently accurate respective positioning of the two magnetic cores, leading to an error-prone gradient measurement, particularly due to the aforementioned parallelism deficiency. The generally adopted solution consists of making the most stable possible assembling and then correcting by calibration or fitting magnetic masses the said alignment deficiency, which represents a source of complexity and lack of fidelity in the measurements.
Moreover, it is known to measure a magnetic field by means of a so-called optical magnetometer using the Faraday effect. For this purpose, a polarized light beam passes through a magnetooptical medium, which rotates the polarization plane by a value proportional to the value of the magnetic field along the propagation direction. An a.c. magnetic field is superimposed on this direction so as to obtain, after detection, filtering and negative feedback, an automatic measurement of the algebraic value of the component of the field in this direction.
Thus, a ferrimagnetic material layer traversed by a polarized light beam rotates the polarization field by an angle .theta., called the Faraday rotation and given by the relation: EQU .theta.=.theta..sub.o L(h/Hs) (1)
in which .theta..sub.o is a specific rotation coefficient, h is the component of the local magnetic field parallel to the light propagation direction, Hs is the field in the plane of the layer perpendicular to the propagation direction, called the saturation field and L is the length of the layer in the propagation direction.
In order to obtain the measurement of field h, a sinusoidal field h(t)=h.sub.o sin .omega.t in the propagation direction is superimposed on the field to be measured. By detecting the light transmitted across a crossed analyzer with the incident polarization, an electrical signal is obtained, whose fundamental is proportional to h.
The magnetooptical effect described by relation (1) assumes that the magnetooptical layer is relatively thick and specifically a few dozen microns, so as to remain within the approximation of the geometrical optics.